Habitats: rock pool

Rock pools are natural aquariums left behind
by the tide. They allow us to see what lives in the sea. For studying rock
pools, one does not need to be able to dive. It is very interesting. You
need to be very careful not to cause any damage to the organisms living
there. What is life in a rock pool? What lives in a rock pool?

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IntroductionAs the tide recedes, not all of the coast is left behind dry. Depending
on the type of rock, one can find shallow or deep rock pools. They offer
better survival chances for animals and plants that need to be submerged
all the time. Because each rock pool is different, they invite different
communities. Deep rock pools near the low tide, have few survival problems,
whereas shallow rock pools near the high tide, have many. Thus the pool's
placing on the shore as well as its depth and size are important factors.

For the snails that are able to survive in between high and low tide
(periwinkle, nerita, melagraphia, cats eye) a rock pool is not necessarily
a better place because their predators are found there (dark rock shell,
white rock shell, trumpet shell). Large fish and octopus may find the rock
pools too small, lacking oxygen for breathing. Rock pools may collect fresh
water during rain storms, which is worse for shallow rock pools high up
the shore where organisms must wait longer for the tide to return.

Warning! rich rock pools are
rare. Treat them with care!

Many small fishes use the rock pool as a shelter against the low tide,
but as soon as the new tide moves in, they leave the pool to wander around
their much larger territories outside the pool, where they find their daily
food. Recent scientific research has shown that most of these small fishes
stay in the same pool all their lives. It is important to these fishes
that they can hide inside the pool, in a crevice, under an overhang or
under a large stone.

This diagram shows four different rock pools. No 2 is a shallow
high tidal pool in Leigh, where very few species survive: slimy green algae
and tiny Stiliger sea slugs. Pool 3 is an upper mid littoral splash pool
in which the neptune's necklace seaweed survives. Pools 4 and 5 are deep
lower mid littoral pools from Ocean Beach, near Whangarei Heads. They are
rich pools with many surviving plant species and sensitive species such
as Cystophora torulosa and Pterocladia capillacea.

Living conditionsIf a rock pool is always covered by the sea, life there would be easy.
The temperature remains stable, and also salinity (saltiness). Once the
sea's blanket is taken away, the rock pool is isolated as if it were a
small aquarium and its living conditions begin to change. Obviously, the
larger and deeper the aquarium and the shorter time it is left in this
state, the better it is for its inhabitants. So deep, large pools near
the low tide are the most interesting ones to study. But every type of
pool, even the small, shallow ones near the high tide, tell us something
about the organisms that are able to survive there.

TemperatureDuring
the day, rock pools are heated by the sun and warm up, but during the night
they are cooled. When water warms up, it does not pass the warmth downward
easily. A thermocline develops, which is a sharp boundary between the warm
water above and the cool water deeper down. It may explain some of a rock
pool's zoning.
The graph shows the rate of warming for small to large rock pools.
Notice how the small pool warms up more so than the large ones, but all
even out at a maximum for the day. The temperature also depends on how
much water evaporates by the wind blowing over the pool.

Here in New Zealand, the difference between winter and summer temperatures
in the sea is some 6-8 ºC but the difference between summer and winter
air temperatures is much larger. A shallow rock pool can be heated to 40
ºC, a temperature that kills most shore creatures.

SalinityAs water evaporates from shallow pools, it leaves the dissolved minerals
behind in the remaining water. The sea's most common mineral is salt. So,
every rock pool becomes saltier as its water evaporates, but shallow pools
towards the high tide, more so than others. Some pools show salt crusts.
Very few if any animals or plants can live there.

When it rains, fresh water drains into the rock pools and depending
on its volume, a rock pool can become too fresh for marine life. Fortunately,
fresh water floats on top, being lighter than salt water. Because it mixes
only when stirred vigorously, animals and plants can often survive deeper
down.

OxygenAnimals
need oxygen for breathing, but plants also need oxygen at night when they
cannot make their own. So the low tide at night is the most critical. Fortunately
most animals are inactive at night, using the least possible amount of
oxygen.
During the day, the plants may produce more oxygen than is used by
the animals and the oxygen level in the pool may rise, rather than fall.
See the graph on the right.

SedimentSediment makes the water murky, depriving plants from the light they
need. Sediment (like mud) also settles on the fine gills of all water breathing
animals, making them sick or suffocating them. When a lot of mud or sand
regularly washes into the rock pool, animals and plants are smothered and
the rock pool looks barren (See photo).

LightLight enters the pool on an angle from above. Deep pools may show a
distinctly shaded side, inviting a variety of red seaweeds. Narrow, steep
pools may not get as much daylight as wide ones, inviting shade-tolerant
species to live near the bottom. Underneath stones and inside crevices,
where it is dark, one may find animals such as sponges and seasquirts,
that would otherwise not be able to compete with the plants on the sun-lit
surfaces.

PeoplePeople like rock pools, for bathing in or for studying. Nowadays so
many people visit the sea that the available rock pools are visited many
times a year. Even when visitors are very careful, the damage to these
often fragile organisms, may accumulate and become visible. So take utmost
care when visiting rock pools, particularly when turning stones.

Remember that a one off disaster can be more damaging than average unfavourable
conditions.

These rich, deep rock pools in the Goat Island marine
reserve never fail to amaze.

These barren rock pools at the Long Bay marine reserve, are
too close to the beach and get covered in sand too often. Nothing can grow
here.

In a large rock pool near Muriwai on the West Coast, large
Isocradactys
anemones in various colours have found their home.

During a rocky shore study, students listen to a scientist
as they explore the shore near Goat Island. By turning stones, some organisms
are killed.

This shallow high tidal rock pool warms up and is sought
after by children. The organisms here get crushed and trodden on. Only
the hardiest can survive or those that grow and reproduce fast.